It is well known that in the case of some of the infective diseases the survival of one attack causes a certain immunity to further attacks. This applies to most of the specific fevers, such as small-pox and scarlet fever. In most diseases of this kind (the Exanthemata) no microbe has been discovered, but in typhoid fever, which is one of the exanthemata and is subject to the same rule of immunity, the microbe is a specific bacillus.

The explanation of acquired immunity is probably the same as in immunity induced by vaccination.